The availability of small, low cost, and relatively powerful microprocessors has resulted in these devices being used a variety of new ways. Previously, the higher cost of microprocessors meant they would be employed only for more complex tasks. However, the commodity market for cheap microprocessors has allows these devices to be used as substitutes for conventional or special purpose circuits, often at equal or lower cost. At the same time, the device's processing power allows them to handle additional functions in a particular application that would be much more difficult to implement using conventional circuits.
In the fields such as distributed control and process management, the availability of cheap microprocessors allows more complex interactions between distributed devices. For example, there are numerous different electrical functions that are initiated at various places throughout an automated system. These functions may include activating mechanical devices, sensing physical quantities, accepting user inputs/controls, detecting system failures and improper states, etc. Conventional approaches require one or more conductors to be provided for each of these functions. For example, a separate wire may connect each temperature gauge in a system with an associated temperature sensor. In environments such as automotive and aerospace, where space and weight are at a premium, such wiring requirements can severely restrict the functionality that can be provided by conventional approaches.
By making use of the previously mentioned microprocessors, the number of conductors needed to provide inter-device communication is greatly reduced. Instead of dedicated wires between related components, a single wire may provide a serial signal path that is used for all inter-device communications. A power supply and return wire may also be connected to some or all of the components. Each device includes a communications node that can send messages to and receive messages from the other devices' nodes on the signal path. Each node receives every message on the signal wires and uses the messages appropriate for operating its associated device.
One problem using a single serial signal path is that of message collisions, where two or more nodes send messages on the signal wires at the same time. One solution to resolving message collisions is provided by the Controller Area Network (CAN) system, which uses bit-wise arbitration. In the CAN system, messages are encoded using signals that represent either dominant or recessive bits. If a communicator sends a dominant bit signal, the dominant bit is present on the signal path regardless of the number of other communicators that are sending recessive bits. Each communicator senses the signal on the signal path, and ceases sending its message if, when sending a recessive bit, the node senses a dominant bit. This process of detecting collisions at each node and ceasing sending messages upon detecting of collisions is referred to as arbitration.
The leading bits of a message in a CAN-type system act inherently as a priority during arbitration. Messages that have the largest sequence of dominant leading bits will win arbitration over other simultaneously transmitted messages. Therefore, the system designer can ensure messages have the desired priority by forming messages having a particular ordering of leading bits in relation to other messages transmitted on the system.
In order to synchronize nodes on a CAN-type system, each receiving node must adjust its internal timers with each received bit so that the receiving nodes stay synchronized with the transmitting nodes. However, it was recognized that for low data-rate systems, such an elaborate synchronization mechanism could be avoided if a commonly accessible timing signal could be used to synchronize the nodes.
For example, in ENVIRACOM® systems provided by Honeywell®, each bit transmitted on the serial line is synchronized to a half cycle of AC power. Where the AC line frequency is 60 Hz or 50 Hz, this provides data rates of 120 bits per second or 100 bits per second, respectively. Because ENVIRACOM is primarily intended for use in residential Heating, Ventilation, and Air Conditioning (HVAC) systems, these low data rates are not an impediment to providing useful system controls. This simplified method of synchronizing control/sensing nodes allows creating relatively sophisticated HVAC systems using legacy thermostat wiring and relatively low-cost HVAC components.
However, as systems become more complicated, the need for higher data rate bus speeds in ENVRACOM-type systems is becoming apparent. However, such systems need to retain backwards-compatibility with previous devices whose data rates are synchronized to the power line frequency. Such systems should also retain compatibility with the higher layers of the message-exchange protocols so that previously designed application software can be reused with the higher data rate systems.